The invention relates to a pumping system for a leak detecting device which operates on the counter current principle with a first turbo molecular pumping stage, to the inlet of which a test piece or a test recipient can be connected and with a second turbo molecular pumping stage, to the inlet of which a test gas detector is connected. Both turbo molecular pumping stages are disposed on a common shaft and have a pre-vacuum pump assigned to their outlets.
The test pieces can be tested for leakage by evacuating the test piece and spraying it from the outside. It is known to fill a smaller test piece with test gas and to place it in a recipient which is subsequently evacuated. If the test gas penetrates the walls, the test piece is not leakproof. The use of a test gas detector which is connected to the evacuating pipe permits detecting whether the test piece is leaky or not. Usually, a mass spectrometer serves as a test gas detector which is calibrated for the mass of the test gas; in the case of helium, it is calibrated detection. It is of particular disadvantage that this helium partial pressure is subject to irregular deviations so that it cannot be eliminated as a constant when the measuring values are evaluated. Presumably, these often strong, irregular deviations of the helium partial pressure are the result of bubbles formed around minute leakages at the oil-sealed vacuum pump; the bubbles are eventually released and move to the inlet side of the pre-vacuum pump where they burst. Furthermore, after a particularly large leak, there is a disturbance in the area around the pre-vacuum pump which affects the sensitivity. This disturbance results from the fact that a part of the relatively high helium pressure is released in the oil of the pre-vacuum pump. The gradually degassing helium causes a helium partial pressure increase in the inlet area of the pre-vacuum pump; this increase can be so high as to falsify the immediately following measurements. Therefore, long recovery times cannot be avoided using leak detecting devices with pumping systems of the kind described.